The present invention relates to a method for electrolytically and continuously treating a hot- and/or cold-worked metal material, particularly a continuously formed one in stainless steel. The treatment method for the material involves removing continuously, in one stage, a surface layer of one or more mixed metal oxides, usually spinel, which surface layer is thicker than the passivation layer which occurs on the surface of stainless steel material, at the same time as a polished surface is obtained. According to the invention, the character of the surface obtained can also be selected by varying a number of control parameters.
When manufacturing continuously formed metal products of thin dimensions, i.e. primarily strip, from a stainless steel material, hot rolling is generally carried out followed by the final stage, cold rolling. The operation in that case is first hot rolling of the material at high temperature, at which the material softens and can be rolled to a thickness of around 5-25 mm. Owing to the high temperature, an oxide scale of a thickness of around 50-500 micrometers is formed on hot rolling, which scale consists of a mixed oxide, usually spinel, comprising at least iron and chromium. Spinel is conventionally defined as AB2O4, A being magnesium, iron(II), zinc or manganese or a combination of these, and B being aluminium, iron(III) or chromium. In the present case, the spinel formed during hot rolling is often of the type Fe(FeCrxOy), where x=1-2 and y=2-4 and the chromium content is often up to 40%. Under the oxide scale formed is a chromium-depleted zone with chromium contents down to 10-12% or even lower. Before the material is worked further, the oxide scale has to be removed. A pretreatment can be carried out for this purpose, consisting of annealing at around 1000-1200xc2x0 C., which softens up the surface, followed by cooling and then blasting which breaks up the oxide scale. After this, a pickling process is normally used, which is an electrolytic treatment stage in an electrolyte consisting of one or more mineral acids or a neutral solution and an ensuing chemical stage. What happens is that the electrolyte/acids penetrate down into cracks in the oxide scale and dissolve the chromium-depleted zone, at which the oxide scale loosens. Following rinsing, the material has a dull, so-called pickled surface. To produce thinner dimensions, the treatment continues with cold rolling, at which the material becomes hard and brittle due to the formation of martensite. To restore the correct material attributes, stress-relieving annealing at around 1000-1200xc2x0 C. follows, at which however a surface layer again appears of a mixed oxide of the spinel type, this time with a thickness of around 10 micrometers. Since the oxide does not have the right stainless properties, this is also pickled away in the same way as in the earlier process. Following rinsing, the material has a dull, pickled surface. In certain cases, e.g. if further forming operations are to be carried out, the pickled surface. In an advantage, but often a bright surface is desired instead, which is then produced by means of bright annealing in a reducing atmosphere, followed by smoothing rolling with only around 2% reduction of the material thickness. Blasting is not used on cold-rolled material, as the surface is destroyed by this. Instead, so-called neolyte treatment is used, which involves an electrolytic treatment with direct current, usually in sodium sulphate, at which chromium(III) is oxidized to chromium(VI), which is soluble.
A number of methods of pickling have been known for a long time. One such relatively modern method is described in SE-A-9301591-5. In this method, the electrolyte bath consists of mineral acid, for example sulphuric acid, or mixtures of mineral acids and the electrolysis is carried out by means of alternating current or alternately direct current at relatively high current densities, 150-250 A/dm2. The patent also covers the special equipment which is used for the method. When pickling according to SE-A-9301591-5, a dull, pickled surface is produced, as in all known pickling methods, for which reason a subsequent polishing stage consisting of bright annealing and smoothing rolling has to be carried out.
When manufacturing products which are not rolled or continuously formed, i.e. piece products, e.g. pipe parts, tapping cocks, bolts, screws or vessels, in stainless material, a polishing process called electro-polishing is usually performed. This process is a batch-wise electrolysis using direct current in e.g. sulphuric acid and phosphoric acid at relatively low current densities, around 10 A/dm2. The process also takes a relatively long time, often up to 10-20 minutes, in spite of the fact that the oxide layer which is being treated is very thin, around 1-5 nanometers. This layer is the passivation layer which always occurs on the surface of a stainless material and consists of chromic oxide, Cr2O3. In the process, the material is made to form an anode and a viscous film is then formed on the surface of the material. This film has a higher density, viscosity and resistance than the electrolyte remaining. As the film adjusts itself to the uneven surface of the material, it is thicker in the recesses than on the crests in the surface. In the recesses, the resistance to the passage of current is higher and the current density therefore lower. On the crests, the current is higher on the other hand and the crests thereby come to be dissolved, resulting in a levelling of the surface of the material.
A number of variants of electro-polishing are described in Sweden""s Galvanotekniska Fxc3x6renings (Electroplating Association""s) xe2x80x9cLxc3xa4robok i elektrolytisk och kemisk ytbehandlingxe2x80x9d (xe2x80x9cManual of electrolytic and chemical surface treatmentxe2x80x9d), p. 263-271, published by Ytforum Fxc3x6rlag. For the electro-polishing of stainless steel, electrolytes are mentioned consisting of sulphuric acid, ortho-phosphoric acid and water or orthophosphoric acid, glycerol and water. The current density is 7-25 A/dm2 and 7-8 A/dm2 respectively and the time is up to 10 and 15 minutes respectively.
In JP 57-101699/82, a batchwise method of electro-polishing a super-alloyed steel with 60% Ni, 23% Cr, 10% Mo, 3% Fe and 4% Nb is described. The method is used for clock parts and is said to give a figured surface, similar to that which is traditionally obtained by hand grinding. The electrolyte used is a mixture of an aqueous solution of phosphoric acid and an aqueous solution of sulphuric acid in a weight ratio of between 9:1 and 1:1. The electro-polishing takes place over 10 seconds-20 minutes, at a current density of 1-50 A/dm2 and a temperature of 15-50xc2x0 C.
No continuous method of simultaneously pickling and polishing continuously formed, stainless material is known, as far as the inventors of the present method know. A major disadvantage of the known pickling process for the removal of a surface layer of mixed oxide of the spinel type is that the surface produced remains dull and pickled, for which reason a subsequent stage with bright annealing and smoothing rolling has to be carried out for a highly bright surface to be obtained. Hitherto, however, no-one has thought it possible to obtain a highly bright surface in one and the same step as pickling in continuous treatment of continuously formed stainless products.
The preconceived notion that highly bright surfaces cannot be obtained in one and the same stage as continuous pickling of stainless material is thwarted by the present invention.
The method according to the present invention is defined in the independent claim 1 and means that a material of stainless steel, which is preferably continuously formed, in particular strip-formed, is treated continuously, an oxide surface layer of a thickness of at least 1 micrometer being removed from the material and the method achieving a polished effect on the surface of the material in the same stage. This pickling in combination with polishing takes place moreover in a time which is considerably shorter than a conventional pickling stage.
It should be emphasized that the invention, in spite of the fact that it is conceived principally to be applied continuously to continuously formed stainless material, in particular strip, but also thread or pipe, is not restricted to this but can be applied to all stainless material in all forms and thicknesses.
According to an aspect of the invention, the surface layer which is removed is a mixed oxide of the spinel type, comprising at least iron and chromium, and has a thickness of 5-1000 micrometers, preferably 10-500 micrometers. If the material which is being treated has been subjected to hot rolling and annealing, but not cold rolling, the oxide layer has a thickness of 30-1000 micrometers, preferably 50-500 micrometers. If the material has also been subjected to cold rolling, an oxide layer which has arisen afresh has a thickness of 1-30 micrometers, preferably 10-20 micrometers. According to the invention, the same method can be used for removing both the somewhat thicker oxide layer of the spinel type which occurs on hot rolling and annealing and the somewhat thinner oxide layer of the same type which arises on cold rolling and annealing. In both cases, polishing of the surface arises in the same stage.
According to another aspect of the invention, the method is carried out by means of electrolysis with direct current, an electrolyte comprising sulphuric acid or salt thereof and/or phosphoric acid and possibly hydrofluoric acid or salt thereof being used. Simultaneous pickling and a polishing effect occur in an electrolyte consisting purely of phosphoric acid, but it is preferred that the electrolyte also contains a certain amount of sulphuric acid, a suitable interval being 0-95% by volume of sulphuric acid and 5-100% by volume of phosphoric acid. The concentrations are most appropriately 2-12 mol/l, preferably 2-10 mol/l and even more preferredly 2-6 mol/l for sulphuric acid and 2-14 mol/l, preferably 4-12 mol/l and even more preferredly 4-9 mol/l for phosphoric acid, while hydrofluoric acid or salt thereof, if used, is added in a concentration of 1-8 mol/l, preferably 2-7 mol/l and even more preferredly 3-6 mol/l. Iron normally also forms part of the electrolyte, in a quantity of e.g. 30-40 g/l. Iron is released from the material on electrolysis, at which it is concentrated in the electrolyte and does not therefore normally need to be added. As for the rest, the participating ions mainly have the functions of providing conductivity (hydrogen ions of the sulphuric acid) or of providing complex binders for iron (fluorine ions, phosphate ions). In addition to the variants of electrolytes already specified, an electrolyte can be used for example which mainly comprises sodium sulphate and sodium fluoride, the contents being suitably 100-200 g/l, preferably 120-180 g/l and even more preferredly 135-165 g/l for sodium sulphate and 10-70 g/l, preferably 20-60 g/l and even more preferredly 30-50 g/l sodium fluoride. Another variant is sulphuric acid and sodium fluoride, suitable contents being the same as indicated earlier for these chemicals. It is also possible to combine the two last proposed electrolytes, the content being mainly sulphuric acid, sodium sulphate and sodium fluoride according to the amounts indicated earlier. Another variant is sulphuric acid and hydrofluoric acid, the concentrations being most appropriately 2-15 mol/l, preferably 4-10 mol/l and even more preferredly 6-9 mol/l for sulphuric acid and 1-10 mol/l, preferably 2-7 mol/l and even more preferredly 3-6 mol/l for hydrofluoric acid. The electrolyte can also according to the invention mainly comprise nitric acid, which however signifies an undesirable negative effect on the environment. Water can be added if necessary, but can also be present in the amount which forms part of the concentrated acids and which arises due to the absorption of atmospheric humidity. The skilled man will easily understand that the variants of electrolyte compositions can be varied in many more ways, it being a measure of the skilled man to optimize the concentrations.
According to a further aspect of the invention, the treatment time is 30 sec-5 min, preferably 1 min-3 min and even more preferredly around 2 minutes when the starting material is hot-rolled and annealed, or 2 sec-2 min, preferably 5-90 sec. and even more preferredly 10-60 sec. when the starting material is cold-rolled and annealed, with the treatment being able to be carried out in one stage or divided into two or more stages. The anodic current density during electrolysis is 0.1-3 A/cm2, preferably 0.3-2.5 A/cm2 and even more preferredly 0.5-2 A/cm2, and the temperature is 50-100xc2x0 C., preferably 60-90xc2x0 C. and even more preferredly 65-80xc2x0 C.
According to another aspect of the invention, the process can be controlled with the aid of the parameters time, current density, temperature, type of electrolyte, equipment type, material type and desired surface. The stainless types of material can be alloyed with for example chromium, nickel or molybdenum. They can be ferritic, martensitic, duplex, austenitic or superaustenitic, for example. The composition for these stainless types of steel is defined in xe2x80x9cStainless Steel, The New European Standardsxe2x80x9d, 2nd Edition, 1997-02, Avesta Sheffield, but other types of stainless steel can also be treated according to the invention, probably even those which have not yet been developed. The desired surface can be varied from normally dull, i.e. a surface of the same character as is obtained by conventional pickling, to mirror-bright or even bright but grainy, Depending on the material which is being treated and the surface which is desired, the parameters time and current density are selected so as to achieve the desired result. In general, Faraday""s Law applies, meaning that the result remains the same if the factor time times current density is kept constant. This theory is not always correct in practice, but as a guideline the factor time times current density is reproduced in the following table in the unit As/cm2 for different materials and different desired surfaces. It is assumed here that a preferred electrolyte with 5 mol/l sulphuric acid and 8 mol/l phosphoric acid is used for electrolysis with direct current, at a temperature of 70xc2x0 C.
According to another aspect of the invention, the treatment stage is executed in one or more electrolytic cells lying in series, the material being contact-polarized anodically and made to run in an electrolyte between cathodic electrodes under the influence of a direct current. The term xe2x80x9celectrodexe2x80x9d here and henceforth implies a single electrode or a bundle of electrodes, the latter variant being common in industry and signifying that each electrode bundle functions in practice as one electrode, in that an electrode bundle holds a fixed polarity (cathode or anode). Moreover, the electrodes or electrode bundles may be of different length, with for example a greater total anodic electrode length or a greater total cathodic length.
According to yet another aspect of the invention, the treatment stage is executed in one or more electrolytic cells lying in series, the material being made to run in an electrolyte between electrodes lying in series, under the influence of a direct current with alternating polarity, every other electrode being anodic and every other being cathodic and each electrode being matched by an electrode of the same polarity on the opposite side of the material.
According to another aspect of the invention, the treatment according to the invention can be preceded and/or followed by a chemical surface treatment with mixed acid in one or more cells, preferably using nitric acid and hydrofluoric acid.
An advantage of the method according to the invention is that treatment takes place very quickly. The treatment is 2-10 times faster than conventional pickling and 10-20 times faster than conventional electro-polishing. This means that existing installations, where space is limited for example by an existing annealing furnace and at the other end by existing rinsing equipment for rinsing, can be given an increased line speed in spite of the limited space. The material can thereby be conveyed at a line speed of at least 5 m/min, preferably at least 50 m/min, even more preferredly at least 60 m/min and most preferredly at least 80 m/min.
The invention has at least the following advantages compared with conventional pickling. The method is faster at the same time as two measures are executed in the same step, namely the removal of oxide and achieving a bright surface. The surface cutting is equivalent to or somewhat less than in conventional pickling, for which reason a higher yield can be obtained. The method facilitates a high level of process control and involves relatively little effect on the environment, at least if nitric acid is avoided. The electrolyte chemicals are relatively cheap, especially sulphuric acid, but also phosphoric acid, which commands around half the price of nitric acid. Compared with bright annealing and smoothing rolling, the method means that the investment costs are halved.